The present invention relates to determining the amount of solids present in a slurry. This determination is necessary in the operation of certain types of air-quality control systems in which a slurry is sprayed into fume-laden gases in order to remove certain pollutants. It is necessary for proper operation that the percentage of solids in the slurry remain within predetermined limits.
In the past, the percent-solids determination was made by measuring the density of the slurry. It was found that this method was not entirely satisfactory because the specific gravity of the slurry is affected not only by the presence of suspended solids but also by solids dissolved in the liquid. If the effect of dissolved solids were ignored, large inaccuracies would result because small changes in the specific gravity of the liquid portion of the slurry could appear to be large changes in the percentage of suspended solids. Thus, it was necessary to calibrate the instruments repeatedly in order to take into account the change in specific gravity of the "process liquid."
This recalibration problem was solved by Schuetz (U.S. Pat. No. 4,047,891), who suggested employing the process liquid in a bubbler pressure sensor in order to eliminate the effect of a change in process-liquid specific gravity. In one of the normal methods of testing specific gravity, the difference in pressure between two elevations in a container of liquid is sensed. The pressures are transmitted to a differential pressure sensor through two fluid lines through which fluid is continually fed into the container. Schuetz suggested using the process liquid as the fluid, and this had the result of virtually eliminating the effect of the process-liquid specific gravity on the percent-solids measurement. As can be seen in the drawings in the Schuetz Patent, which is hereby incorporated by reference, the process liquid used in the bubbler is taken from a thickener tank. The thickener tank is located outside the reaction tank in which the slurry that is to be used in the scrubber is contained. The process liquid from the thickener then flows through appropriate valving to the reaction tank, and in doing so it serves as the medium through which the pressure to be sensed is transmitted.
The present invention is an improvement over the Schuetz method in that the method and apparatus of the present invention permit the measurement to be carried out more simply. In addition, they avoid the possibility that operator error could permit the slurry to back up through the wrong tubes and thereby damage equipment. Furthermore, they ensure that the properties of the process liquid used in the measurement are as close as possible to those of the process liquid that is actually in the reaction tank.
The present invention is a method and apparatus for measuring the suspended-solids content of a liquid in which the suspended solids have a tendency to settle. A vessel for containing liquids is provided, as are means for agitating liquid contained in the vessel. The agitating means are provided so that any solids in the liquid remain in suspension. A tube extends into the vessel interior, and the tube is hollow and open at its lower end so that it will contain some liquid when the vessel contains liquid up to a level high enough to reach the tube. This tube isolates the liquid contained in it from the action of the agitation means, so the solids in the tube are allowed to settle out. The difference between the pressures at two points of equal elevation, one inside and one outside the tube, is sensed, and this gives an accurate indication of the suspended-solids content of the liquid in the vessel.